Rubber vulcanization



Patented July 4, 1950 UNITEDSTATES PATENT OFFICE RUBBER VULCANIZATION George E. P. Smith, Jr., Akron, Ohio, assignor to The Firestone Tire & Rubber Company, Akron, Ohio, a corporation of Ohio No'Drawing; Application April 10,1944, Serial No. 530,393

The objects of the invention include providing a new class of chemical compounds which are valuable delayed-action accelerators oft'he vulcanization of a rubber, providing a new rubber composition whichis safe for factory processing but which vulcanizes extremely rapidly at conventional rubber vulcanizing temperatures, and providing a new vulcanizedrubber article hav- 'ing improved physical and chemical properties. The above and further objects will be manifest in the description of the invention which follows. As a part of the'present invention, it has been discovered that members of a new class of chemical compounds are very effective in accelerating the vulcanization of natural or synthetic rubber. The new compounds are sul- 'fenamides of the following general formula:

wherein R is hydrogen, an 'alkyl radical or an alkoxy radical, and R1 is a hydrogen atom, an alkyl radical or a cyclo-alkyl radical. the new compounds are believed to be derivatives of sulfenamide (HSNI-Iz). Examples of alkyl radicals include methyl, ethyl, .propyl, butyl, amyl, hexyl, heptyl, octyl, decyl, lauryl, benzyl, and similar radicals. Examples of cycloalkyl radicals include cyclohexyl and methylcyclohexyl radicals.

Examples of com-pounds of the invention in- .clude: N-cyclohexyl-tetrahydrobenzothiazyl-sul- Thus,

Example 1 Preparation of the new compound, N-cyclohexyl-tetrahydrobenzothiazyl-sulfenamide, was accomplished according to the following series of reactions:

The intermediate product, Z-mercapto-tetrahydrobenzothiazole, was prepared by adding 14A grams of 2-chlorocyclohexanone to 12.0 grams of freshly prepared ammonium dithi'ocarbamate, together'with a little ether. The reaction was not violent, and. the mixture was allowed to stand .15 .hours at room temperature. The ether was removed by evaporation, leaving a brown. solid. 0n extracting this solid with boiling benzene there were obtained 6L8. grams of the mercaptan.

On re'crystallizing' the product from benzene a white solid was obtained melting at l74-17I6 C.

The sulfenamide was prepared by oxidizing with iodine .(in aqueous potassium iodide.) an aqueous solution containing 10.0 grams of .the mercaptan (prepared as above) 4.6 grams ofsc- .diumhydroxide and 23.0 grams (4 moles per mole of mercaptan) of cyclohexylamine. The mixture was stirred strongly and cooled to 5-10C. during the reaction. A light tan precipitate of the N-cyclohexyl tetrahydrobenzothiazyl sulfenamid'e was produced weighing 9.7 gramsa yield of 62%. A portionof the sulienamide recrystallizedfrom a mixture of. ether and petroleum ether was a white solidmelting at -103'C.

Example 2 N-isopropyl tetrahydrobenzothiazyl sulfenamide was prepared in a similar manner from 8.0 grams of 2-mercapto-tetrahydrobenzothiazole (prepared as in Example 1) and a large excess (15-20 moles per mole of the mercaptan) of isopropylamine. The mixture of reagents was oxidized in cold aqueous alkaline solution by the dropwise addition of iodine-potassium iodide solution and agitation of the reaction mixture. The desired sulfenamide precipitated as a yellow solid, which was separated from the reaction mixture after no further precipitation occurred on adding further iodine solution. The solid product melted to a light yellow oil on warming to room temperature. The sulfenamide was obtained in a yield of 5.7 grams or 53% of the theoretical.

The sulfenamides prepared according to Examples 1 and 2 were tested as rubber vulcanization accelerators by comparing them to the standard accelerator, mercaptobenzothiazole, in the following rubber formula:

Ingredients: Parts by Weight Rubber (Smoked sheets) 100.00

Sulfur 3.00 Zinc oxide 5.00 Stearic acid 1.10

Accelerator 0.75

Samples of the three rubber compositions, so formulated, were heated for 30 minutes at 240 F. to-determine whether or not they possessed any tendency to pre-vulcanize or set-up during processing operations prior to the usual vulcanizing step, and the following physical testing data were obtained:

Other samples of the same rubber compositions were heated for 20, 40 and 60 minutes at 280 F. to test the accelerating ability of the sulfenamides at a conventional rubber vulcanizing temperature, and the following data resulted:

4 I comparison with mercaptobenzothiazole, and the physical properties were even higher after the longer heating periods.

Other members of the preferred group of sulienamides coming under the general formula set out above, such as, for example, those specific sulfenamides 1 enumerated above, are also safe, delayed-action accelerators which produce very rapidly vulcanizing rubber compositions.

The various members of the new class of chemical compounds are equally effective delayedaction vulcanization accelerators in rubber tread compounds, latex compounds or other conventional types of vulcanizable compounds of rubber, reclaimed rubber, or synthetic rubber, and consistently produce rapid-curing compositions, which in the vulcanized state possess unusually high modulus, tensile and abrasionresisting properties. The various synthetic rubbers vulcanizable by heating with sulfur are herein considered equivalent to rubber, examples being butadiene polymers, isoprene polymers and various interpolymers of conjugated diolefins and vinyl compounds, such as Buna S (copolymer of butadiene and styrene) and Buna N (copolymer of butadiene and acrylonitrile) Although sulfur has hereinabove been mentioned as the preferred vulcanizing agent, other known vulcanizing agents susceptible to acceleration are contemplated.

The preferred class of sulfenamide derivatives may be prepared by other methods than those mentioned above. For example, a mixture of a 2-mercapto-tetrahydrobenzothiazole and ammonia or a suitable primary amine may be oxidized by hydrogen peroxide or other oxidizing agent, such as chlorine, bromine, hypochlorites or other hypohalites. The invention is not limited to derivatives prepared in accordance with any particular method, but includes the preferred class of compounds, however they may have been formed.

In addition to the above-enumerated desirable properties imparted by the new accelerators to rubber compositions, it has been found that the vulcanized compositions also are highly eflicient, having low hysteresis characterics. When compared to conventionally accelerated vulcanized rubber compositions, the new compositions are found to be unusually resistant to deterioration upon flexing or aging.

Conventional accelerator activators may be em- Modulusof Elasticity Tensile Strength Accelerator m lbs/1n. at elonin lbs/in. at

gation of 600% break Minutes. 20 40 c0 '20 4o 6 Mercaptobenzothiazole 650 650 600 2,550 2,700 2, 750 N-Oyclohexyl-tetrahydrob enzothiazyl sulfenamide ,100 1,9 0 ,050 3, 75 3,300 3,775 N -Isopropyl tetrahydrobcnzothiazyl sulfenamide 825 1,350 1,375 2, 900 3,350 3, 550

The two sets of testing data given above show that the sulfenamides tested are excellent delayed-action accelerators, since the rubber compositions containing them did not vulcanize at the relatively low vulcanizing temperature of 240 E. (which is often reached during factory processing of a rubber composition), in contrast with the composition including the standard accelerator, mercaptobenzothiazole. Furthermore, the new accelerators imparted very high physical properties to the rubber compositions after the relatively short cure of 20 minutes at 280 F'.', in

utilized in the range of 0.1 to 10 parts per 100 parts of rubber.

Modification may be resorted to and obvious chemical equivalents substituted in the specific examples of the invention without departing from the spirit or scope of the invention as defined in the appended claims.

What is claimed is:

1. The method of vulcanizing rubbe which includes heating a rubber and sulfur in the presence of N-cyclohexyl tetrahydrobenzothi azyl-sulfenamide.

2. A vulcanizable rubber composition including sulfur and N-cyclohexyl-tetrahydrobenzothiazylsulfenamide.

3. A vulcanized rubber product prepared by heating a rubber and sulfur in the presence of N- cyclohexyl-tetrahyd.robenzothiazyl sulfenamide.

4. The method of vulcanizing rubber which includes heating a rubber and sulfur in the presence of N -isopropyl-tetrahydrobenzothiazyl-sulfenamide.

5. A vulcanizable rubber composition including sulfur and N-isopropyl-tetrahydrobenzothiazylsulfenamide.

6. A vulcanized rubber product prepared by heating a rubber and sulfur in the presence of N-isopropyl-tetrahydrobenzothiazyl-sulfenamide.

7. The method of vulcanizing rubber which includes heating a rubber and sulfur in the presence of a sulfenamide of the formula wherein R is a member of the group consisting of hydrogen, alkyl radicalsand alkoxy radicals, and R1 is selected from the group consisting of hydrogen, alkyl radicals, and cycle-alkyl radicals,

8. A vulcanizable rubber composition including sulfur and a sulfenamide of the formula wherein R is a member of the group consisting of hydrogen, alkyl radicals and alkoxy radicals, and R1 is selected from the group consisting of hydrogen, alkyl radicals, and cyclo-alkyl radicals.

GEORGE E. P. SMITH, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 19,286 Zaucker et a1. Aug. 21, 1934 2,196,607 Mathes Apr. 9, 1940 2,260,380 Jones Oct. 28, 1941 2,261,024: Hanslick Oct. 28, 1941 

7. THE METHOD OF VULCANIZING RUBBER WHICH INCLUDES HEATING OF RUBBER AND SULFUR IN THE PRESENCE OF A SULFENAMIDE OF THE FORMULA 